Changed "echo > stamp-h.in" in Makefile.in to "date > stamp-h.in" so that

stamp-h.in will be different each time configure.in and related files are
changed, making it easier to check it in (which is necessary to prevent
unnecessary autoconf calls).

My declaration of initgroups() in slocal.c to eliminate the "no prototype"
warning wasn't portable (FreeBSD 3.[23] choked).  Now use AC_EGREP_HEADER to see
where initgroups() is declared, if anywhere.